As shown in FIG. 51, such copying machines, that is, electrophotographic apparatuses, which have a pair of upper and lower heat rollers 914a and 914b installed in the fixing device 914, are commonly known in the art. The copying machine of this type is capable of executing both the mono-color copying operation and the full-color copying operation. In the copying machine, a scanning is made of an original document (not shown), which has been placed on the document platen 903, by the light source lamp 902 of the optical system 901, and the photoreceptor 905 is exposed by the reflected light from the original document that is directed thereonto through a filter 904. A latent image thus formed on the photoreceptor 905 is developed by color toner stored in developing devices 906 through 908, or by black toner stored in a developing device 909. Then, the toner image is transferred onto a sheet of paper that is supplied from one of the paper cassettes 911 through 913 by the use of an intermediate transferring belt 910, and this sheet of paper is sent to the fixing device 914. In the fixing device 914, the upper and lower heat rollers 914a and 914b are pressed against each other to form a fixing section, where the toner of the toner image melts down and the toner image is thus fixed onto the sheet of paper.
In the copying operations, there are two copying modes: a mono-color copying mode for making mono-color copies that do not need any gloss on their picture; and a full-color copying mode for making full-color copies that need gloss on their picture image. Here, depending on the respective copying modes, different fixing conditions are required in relation to materials of the heat rollers, fixing temperatures, etc.; however, the conventional copying machines, which are provided with the fixing device 914 having only the two heat rollers 914a and 914b, have failed to fully satisfy all these requirements. This results in a problem of limited materials of copy paper to be used, and it has been difficult to obtain superior copied images.
In order to solve such problems, it has been suggested to install different fixing devices respectively used for the full-color copying mode and the mono-color copying mode. This arrangement makes it possible to provide optimum copying operations for both the full-color copying mode and the mono-color copying mode, thereby reducing the cost of copying in the mono-color copying mode. However, such an arrangement having two individual fixing devices causes the structure to become more complicated as well as causing the apparatus to become bulky. This makes it difficult to adopt the arrangement.
Meanwhile, Japanese Laid-Open Patent Applications No. 98036/1976 (Tokukaishou 51-98036, U.S. Pat. No. 3,965,331) and No. 191979/1990 (Tokukaihei 2-191979, U.S. Pat. No. 4,928,148) disclose a fixing device wherein: three heat rollers, the upper, middle and lower ones, are provided in a contacted state; the first fixing section is formed between the upper and middle heat rollers while the second fixing section is formed between the middle and lower heat rollers; and a conveyer belt for transporting sheets of copy paper to the first and second fixing sections is arranged to move up and down in response to either of the fixing sections to be used.
In accordance with this arrangement, different fixing conditions are obtained by selecting materials of the upper, middle and lower heat rollers appropriately and providing different set temperatures to the first and second fixing sections; thus, the fixing operations are conducted using the different fixing conditions that are respectively suitable for the full-color copying mode and the mono-color copying mode without being limited by materials of copy paper to be used. Further, in comparison with the case of installing individual fixing devices used for the full-color copying mode and the mono-color copying mode, this arrangement achieves such features as: easy designs on mechanisms such as a transporting mechanism for copy paper; simplified structure; reduction of the cost; and compactness of the apparatus.
In the fixing device installed in the above-mentioned conventional copying machine, both of the first and second fixing sections are heated and temperature-insulated during the warm-up of the copying machine and during the stand-by state thereof. Here, as disclosed in Japanese Laid-Open Patent Publication No. 191979/1990 (Tokukaihei 2-191979), since the rated power consumption (power to be supplied to the fixing device) of the copying machine is limited, it is designed in its specification to restrict the total electrical quantity, which is required for heating up and temperature-insulating the first and second fixing sections, etc., within the rated power consumption.
However, in this case, it takes a long time from activation of the power switch until the first and second fixing sections are heated up to respective set temperatures, and electric power to be required for the temperature insulation is restricted. Therefore, the copying machine of this type has a problem wherein during the stand-by state between one copying operation and the next, the temperature insulation tends to be insufficient. Further, it has another problem wherein since the upper, middle and lower heat rollers are constantly kept in the contacted state, the efficiency of temperature adjustments to be applied to the heat rollers is reduced, for example, in the case where there are great differences in the set temperatures of the heat rollers that are kept in the tightly contacted state.